The present invention relates generally to a system for calibrating scales. More particularly, the present invention relates to a system for calibrating belt scales.
In conjunction with the sale of many types of bulk materials, it is necessary to weigh the bulk materials to ensure the accuracy of the amount of bulk materials being sold. When very large amounts of bulk materials are sold, such as in ore mining operations, it is typically not possible to place the bulk materials onto a stationary scale for weighing. Conveyor belt scales have been developed for these circumstances that permit the bulk materials to be weighed as the bulk materials are transported on a conveyor belt.
When belt scales are used, it is typically necessary to perform periodic calibrations of the belt scale to ensure that the weight indicated by the belt scale is actually correct. In certain industries, calibration of belt scales is required by law.
One technique for calibrating a conveyor belt scale involves passing a known amount of material over the conveyor belt scale to verify the accuracy of the conveyor belt scale. To verify the accuracy of the belt scale at various weights, it is necessary to run several tests at multiple weights.
One common method of calibrating conveyor belt scales involves placing an accurately weighted length of a roller chain on the conveyor belt and then passing the roller chain over the belt scale. One drawback of this technique is that the weight of the roller chain cannot be varied. As such, the roller chain only ensures accuracy of the conveyor belt scale at the tested weight. Additionally, the roller chains must be calibrated on a yearly basis.
For example, Blubaugh, U.S. Pat. No. 3,396,573, discloses an assembly that has a plurality of interconnected wheeled trucks that each have a known weight. The assembly is attached in a fixed position over the belt scale while the conveyor belt is moving beneath the assembly to calibrate the belt scale.
Jones, U.S. Pat. No. 2,974,518, describes a belt scale calibration system where weights are placed on an elongated frame. The force of the weight on the elongated frame is applied to the conveyor belt through a rotatable wheel. The weight measured by the belt scale is then compared to the weight on the elongated frame.
Wilson et al., U.S. Pat. No. 3,976,150, describes a structure for use in calibrating a belt scale. The Wilson et al. structure includes a frame that extends over the conveyor belt and which is mounted to the belt scale. Placing a known amount of weight on the frame allows the belt scale to be calibrated.
Karpa, U.S. Pat. No. 4,658,921, discloses an apparatus for calculating a stationary scale. The apparatus includes a frame mounted over the scale and a hydraulic press mounted to the frame for applying force to the scale.
McDonald, U.S. Pat. Nos. 3,656,337 and 3,850,023, describe a calibration system in which the conveyor belt is lifted off the belt scale and a calibration load is placed on the belt scale.
Laurent, U.S. Pat. No. 4,309,896, discloses a weighing system that includes at least one flexible body. The flexible body compresses in response to a force placed thereon. Compression of the flexible body causes hydraulic fluid to be transmitted to a pressure transducer. Laurent also discloses the use of smaller flexible body attached to the pressure transducer for calibrating the pressure transducer.